Valve have just confirmed that they will release their Steam-client for Mac on May 12th. It was originally slated for the end of April, but those plans...well, went up in steam (I blame our Swedish editor Jonas Elfving for that pun).
So in a few weeks, Mac-players will be able to pick up Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal and Half-Life over Steam.